HYDROIDA. BALE. 257 



In hip last work — " Systematic Studies on the Sertulariidse " 

 (which only reached me after much of Part I. was in print), 

 Levinsen combats the objections which have been made to his 

 system, and recapitulates and further elaborates his views at 

 considerable length, and with such force and cogency as can 

 scarcely fail to impress any stiident of the Sertulariidse who 

 examines them. Levinsen relies entirely on the opercular 

 apparatus for his generic distinctions, disregarding altogether 

 the colonial characters ; so that he recognises, for instance, no 

 distinction between a Pasythea and a Sertularia, or between 

 a Sertularella and a Selaginopsis, if the opercula are alike in 

 each case. 



Of course, it does not necessarily fallow from the acceptance 

 of Levinsen's views regarding the importance of the oper- 

 culum that his genera must be adopted throughout. Ad- 

 mitting that the colonial characters are of secondary 

 significance, they may still be considered, and undoubtedly 

 will, by some observers, be considered sufficiently important 

 to furnish grounds for generic distinctions. This is evidently 

 the opinion of Dr. Stechow, who has, since the date of Levin- 

 sen's last paper, published a list of the genera of the 

 Hydroida,^ in which, while mainly adopting Levinsen's 

 divisions, he has included such genera as Pasythea, Selaginopsis, 

 and Didyocladium, which Levinsen expressly declares to be 

 merely species of Sertularia, Thuiaria, and Sertularella.. This 

 combination, or intermixture of two diverse systems would 

 logically lead to the further multiplication of genera, as will 

 be apparent if we consider, for example, the characters of 

 Pasythea. The hydrothecse of this genus are not always 

 paired, as Stechow describes them ; in P. quadridentata the 

 paired condition exists, in P. hexodon it does not. I pointed 

 out long ago that according to the colonial characters P. 

 quadridentata is allied to Sertularia, while P. hexodon was as 

 obviously akin to Thuiaria. The same conclusion may be 

 arrived at from the zooecial characters, and Levinsen ac- 

 cordingly classes the former species as a Sertularia, and the 

 latter as a Thuiaria. Now, if we admit, as Stechow does, 

 that the differences between the opercula in Sertularia, 

 Odontotheca, and Thuiaria are valid generic characters, then 

 we must, on the same principle, admit that P. hexodon should 

 be placed in a genus distinct from P. quadridentata, since the 

 difference between these two species is precisely the same 

 as that between Sertularia and Thuiaria. The same principle 

 applies to Didyocladium and Selaginopsis ; obviously a 

 Selaginopsis with an operculum should be placed in a different 



1. Stechow — Hydroidpol3'pen der japanischen Ostkiiste, II., 1913. 



